The present invention relates to processes and plants for parboiling and preparation of rice and to parboiled rice.
Parboiled rice is a rice that has been subjected to steeping, thermal treatment, drying and, often, machining. This treatment permits, in particular, a non-sticky rice to be obtained, the grains of which are intact, but with the drawback that the cooking-time is longer.
The invention overcomes this drawback by virtue of a process for parboiling rice, both paddy rice and cargo rice, that permits parboiled rice to be obtained having a greatly shortened cooking-time.
The process according to the invention consists in steeping the rice in water at a temperature lower than 70xc2x0 C. under such conditions and for such a time that, after removal of the surface water by drainage or spin-drying, steeped rice is obtained having a moisture content by weight greater than 31%. Afterwards a gelatinisation of the steeped rice is carried out by heating it to a temperature greater than 130xc2x0 C. while maintaining the moisture content by weight greater than 20% for a sufficient period to obtain core-gelatinised rice. In accordance with the invention, gelatinisation is carried out by causing the steeped rice to pass continuously into an enclosure so as to limit, by virtue of the equilibrium which is established within the enclosure between the moisture content of the rice which is undergoing gelatinisation and the moisture content of the gaseous atmosphere of the enclosure, the quantity of vapour emitted from the rice during gelatinisation, in order to maintain the moisture content by weight of the rice in the course of gelatinisation at a value greater than 20%, and by heating up the steeped rice in the enclosure essentially by conduction so as not to increase the moisture content by weight of the gelatinised rice appreciably, and in particular so as not to exceed a content of 28%.
By proceeding in this way, two pitfalls are avoided. If the rice is heated up by a current of hot air, drying necessarily occurs which reduces the moisture content of said rice very rapidly, bringing about fissuring of the grains of rice. Moreover, a treatment by a current of hot airxe2x80x94by reason of its necessary brevity if the grain is not to be roastedxe2x80x94is really not very homogeneous because, by reason of the rapidity of the release of the water, certain grains no longer have enough thereof for gelatinisation of the starch to take place properly. If, on the other hand, gelatinisation is carried out by a treatment with steam under pressure, the moisture content by weight exceeds 28% or thereabouts. It is found that this reinforces the texture of the grain that has less porosity. The cooking-time is then very considerable. By confining the rice within an enclosure and by heating it up by conduction, and not by convection with the aid of a hot fluid, in particular by contact with the metallic enclosure which is heated up by passage of current or by an external jacket through which heat-exchanging fluid is passing, the moisture content of the rice can be brought well under control while the temperature thereof is raised in order to gelatinise it.
Steeping is carried out, in general, for at least three hours at a temperature from 40 to 70xc2x0 C., it being possible for the time to be shorter, the more elevated the temperature. The ratio of the volume of the rice to that of the water is at least 2.5. Steeping can be carried out in a vat or in a hopper. The drainage device may be a vibrating filter which ensures separation of the water that is situated on the surface of the grains of rice, constituting, in particular, from 2 to 3% by weight.
In general, the gelatinised rice is dried to a moisture content by weight of 11 to 13% in order to obtain marketable rice, and, if need be, the dried rice is also machined.
The duration of passage or dwell-time of the steeped rice within the enclosure preferably amounts to between four and ten minutes. Above ten minutes, the moisture content of the rice might diminish too much.
It can be ensured that, in a steady state, the rice occupies from 5 to 30% of the volume of the enclosure.
In order to prevent certain grains of rice from remaining continually in contact with the heated enclosure and being roasted, it is preferred to displace the rice in the enclosure by causing the enclosure to vibrate, so that it is never the case that the same grains of rice are in contact with the enclosure.
The invention also provides a process for preparation of rice, which consists in parboiling it by the process according to the invention, in cooling it to ambient temperature (15 to 30xc2x0 C.) while maintaining its moisture content at more than 19%, in machining the cool rice, in reheating it to at least 75-80xc2x0 C., then in flattening the reheating rice and in drying it to a moisture content of 11 to 13%.
At the end of the parboiling process the grains are transferred into a drying column, in order to be cooled therein. In this column the rice is traversed by a current of air at ambient temperature. In this way this rice is brought back to ambient temperature, losing from 3 to 5% of moisture.
The machining is implemented by successive passage through 3 machining cones that are used conventionally in order to blanch rice.
For example, and most often, these appliances are composed of a metallic truncated cone which is lined with an abrasive layer and which rotates within an enclosure constituted by a perforated sheet of exactly the same shape but slightly larger. The rice is introduced between the truncated cone and the metallic housing; it is disposed in a ring, rotating with the truncated cone. The blanching of the grains results from the rubbing of the grains against the abrasive surface and also the rubbing of the grains against one another. In order to implement complete blanching, that is to say the exposing of the albumen of the grain, the grains of rice that have been machined are immediately subjected to heating and flattening.
The moisture content of the rice at the end of the machining amounts to between 17 and 20% of moisture. If the flattening of the grains takes place without prior reheating, by passing between cold metallic rollers, the appearance is observed of numerous cracks in damaged grains, a part of which has been torn out in the course of flattening. The cracks in the product show an increase amounting to 20% by weight. Only 25% of the grains remain intact, not deformed and with no missing part.
The reheating of the grains above their gelatinisation temperature, from 60xc2x0 to 75xc2x0 C. depending on the varieties, permits their plasticity to be increased appreciably by crossing the glass transition temperature of starch. This heating has to be carried out without altering the moisture content of the grains. Heating in hot air, for example, would cause a rapid lose of moisture and a stiff of the grain with appreciable fissuring and cracking.
The use of infrared rays to reheat the grains is entirely appropriate; treated in a monolayer, the grains pass from 25xc2x0 C. to 80xc2x0 C. within 20 seconds without a change in their moisture content. The grains that have been reheated in this way can be flattened without formation of cracks and with a proportion of fissured grains (fissures visible to the naked eye) lower than 10% (number of fissured grains/number of total grains).
The flattening is carried out by passage between two metallic rollers, for example the rollers that are used in order to flocculate cereals. The spacing between the rollers is regulated between 0.2 and 0.5 mm, preferably between 0.3 and 0.4 mm, in order to obtain the effect on the grains that is being sought. The identical speed for the two rollers is regulated between 20 and 60 rpm, preferably between 30 and 50 rpm.
Flattened grains of rice are then immediately transferred into a drying column, where they are traversed by a current of air at a temperature lower than 50xc2x0 C. In this way they are progressively brought back to a moisture content of 11 to 13%, in order to be conditioned afterwards.
The thickness of the grains is measured in the following way.
The cross-section of the grain through its middle causes an ovoid surface to appear.
The thickness E taken into account by the measurement is the smallest section taken through the middle of the grain.
In practice, the measurement of the thickness E is carried out with sliding callipers on the middle of the grain, viewed in profile.
After flattening and final drying, the mean thickness E of 100 grains taken at random is lower by at least 15% than the thickness E of the dry grain that has not been flattened. The flattening will preferably cause the thickness of the grain to diminish by 20 to 25%, it being understood that B should not be lower than 0.9 mm nor greater than 1.3 mm. Below the lower value the grains are too flat, resembling discs, and cannot reasonably be described as grains of rice. For the uninformed consumer they fall into the category of flakes. It is preferred that E amounts to between 1.1 and 1.2 mm.
After flattening and drying, the rice that is obtained exhibits more than 90% of non-fissured grains. The fissured grains are described as cleaved rice; they exhibit one or more fractures which are clearly visible to the naked eye, which are always transverse and which extend from one side of the grain to the other. These fractures are the result of excessive stresses within the grain which are generated by too extreme differences in temperature and/or in moisture between the interior and the periphery of the grains.
The invention also provides a plant for parboiling rice, which comprises, successively, a steeping vat, a drainage device and an enclosure having an inlet and an outlet, means for heating the enclosure and means intended to cause the enclosure to vibrate in order to cause the rice to pass therein from the inlet to the outlet.
The continuous plant according to the invention permits all the grains to be treated in individualised and homogeneous manner and behaves, at the same time, as a heat-exchanger and as a divided gas/solid chemical reactor.
As soon as they have entered the enclosure, the grains of rice will be individually heated by contact with the hot wall of the enclosure. The moisture that is contained uniformly in the grain will be progressively converted into steam as the grain becomes heated up. During this same time the roof of the enclosure will be charged with water vapour and will maintain a moisture content around the grain which limits its dehydration and thus permits complete gelatinisation of the starch.
In order that the treatment proceeds in satisfactory manner and that the grains are completely transformed, the passage-time must permit the temperature of the grain to be raised to 130xc2x0 C.
The dwell-time of the grains in the enclosure is governed by the length thereof and by the frequency of vibration that is applied. The dwell-time is regulated as indicated above, in order to obtain a good containment of the grains of rice within the enclosure, which limits the lose of water and maintains the atmosphere of vapour around the grain.
In one embodiment the invention provides a plant in which there are provided at the outlet of the enclosure, successively, a cooling device, a machining device, a heating device, a flattening device and a drying device.
The invention also provides a batch of grains of parboiled and non-fissured rice which is characterized in that at least 90% of the grains are core-gelatinized according to the alkali test and have a firmness lower than 62 in the Chopin visco-elastograph (alveograph) after being immersed in boiling water for eight minutes.
It is found, in fact, that the grains of parboiled rice according to the invention are gelatinized in a very homogeneous manner and that, in particular, 99% of the grains and even 100% of them are gelatinized and, nevertheless, entirely unexpectedly, the rice that has been parboiled in this way cooks very rapidly.
In addition, if grains having an amylose content by weight lower than 15% relative to dry matter are chosen as grains of rice to be treated, it is found that parboiled rice according to the invention can be cooked in 5 minutes. The varieties of rice that have this amylose content are, in particular, Khao Dawk Mali (Thailand), Miara (France) and Horn Mali (Thailand).
This short cooking-time is also obtained with rice not having this amylose content it said rice is subjected to the process described above by a flattening operation being implemented.
The alkali test is carried out as follows: 100 parboiled grains of rice are placed in a 1.7% solution by weight of KOH for 20 hours.
After 20 hours of reaction the degradation sustained by the grains is observed:
intact grains:non-gelatinised grains
grains dispersed in an opaque chalky mass:only slightly gelatinised grains
completely translucent dispersed grains without chalky opaque core:totally gelatinised grains.
The Chopin visco-elastograph permits the visco-elastic characteristics of the cooked rice to be evaluated by assessing its deformation under the application of a constant force of 700 g for 20 seconds and then relaxation of the force for 40 seconds.
For the measurement in the visco-elastograph, 100 g of rice are cooked in one liter of boiling water. At the end of the cooking the grains are drained for one minute on a sieve with a mesh size of 1.25 mm and are then left to cool for 15 minutes in a 60-mm Petri dish which is placed on a moist sponge plate and covered by an 80-mm Petri dish in such a way that the latter creates a seal with the water which saturates the sponge plate. Three grains that have been cooled in this way are then placed on the visco-elastograph. The thickness E of the grains before application of the force and the thickness e1 after flattening are noted. From these values the firmness F=100. (e1/E) is calculated. Six measurements are carried out in respect of the same cooking, in order to calculate the mean and the standard deviation of the six results obtained.
Moreover, these grains of rice which have been parboiled in accordance with the invention, having an amylose content by weight lower than 15% relative to dry matter, have the following property: the cooked grains, placed in a hermetically sealed receptacle and put in the refrigerator at 4xc2x0 C. for 6 days, have a firmness measured in the visco-elastograph at least equal to 120% of that which they have 15 minutes after cooking. This property is very much sought-after, particularly when a dish has to be prepared in advance.